Japanese Patent Application Publication No. 2003-80969 (JP-A-2003-80969) discloses a driver state detecting device for determining whether a driver is driving inattentively based on the frequency of each direction of the driver's line of sight within a predetermined time in the past and the frequency of each angular velocity of the steering angular velocity within a predetermined period in the past.
In this driver state detecting device, preconditions are set in which, when there are other cars in the neighboring lanes and the driver intends to remain in the same lane and the driver is driving attentively, relatively frequent steering operations occur and the steering angular velocity becomes relatively large because the driver intends to stabilize the position of the vehicle, whereas when the driver is driving inattentively, relatively sluggish steering operations occur and the steering angular velocity becomes relatively small, whereby the driver state detecting device determines whether the driver is driving inattentively based on the frequency of each steering angular velocity of the steering angular velocity within a predetermined period in the past when the frequency of each direction of the driver's line of sight within a predetermined time falls within a predetermined state.
As described above, this driver state detecting device is intended to accurately determine whether a driver is driving inattentively when the frequency of each direction of the driver's line of sight within a predetermined time in the past falls within a predetermined state, by determining whether specific preconditions, which are preset based on statistics of steering angular velocity within a predetermined period, are met.
However, in the driver state detecting device according to JP-A-2003-80969, because specific preconditions on observable data (steering angular speed) for determining the occurrence of inattentive driving are defined, the driver state detecting device cannot respond to steering operations that do not apply to such specific preconditions, and therefore the driver state detecting device may determine that inattentive driving is occurring even if the driver is not driving inattentively, or, on the other hand, the driver state detecting device may determine that inattentive driving is not occurring even if the driver is driving inattentively.